Drivers of 4x4s are putting other motorists at risk by flouting mobile phone and seat belt laws, a study says.

Imperial College London found 4x4 drivers were four times more likely to use mobile phones than other drivers, while a third more shunned seat belts.

The team, who observed 41,000 motorists, said those in 4x4s took more risks as they felt safer in their cars, the British Medical Journal reported.

Another study also cast doubt on claims by ministers roads are getting safer.

The study by Oxford University and the South East Regional Public Health Group in Guildford found when hospital statistics were used instead of police figures serious injuries caused by road crashes had not changed since 1996.

There are still too many people taking unnecessary risks on the roads

Roger Vincent, of the Royal Society for the Protection of Accidents

But the government uses police figures to measure its progress towards reducing road deaths and injuries by 40% by 2010.

These show a drop of nearly a third, leaving the government on course to hit its target.

Lead researcher Dr Mike Gill said: "The discrepancies are likely to be down to under-reporting of accidents to police.

"But it raises questions about the police figures and I think it is important the government looks at the hospital figures."

Road Safety Minister Stephen Ladyman said the study was "misleading".

But it is the study on 4x4 drivers which is set to spark most controversy.

City owners of the vehicles, dubbed Chelsea tractors, have been heavily criticised in recent months because they are considered environmental unfriendly.

Report author Lesley Walker said: "Although 4x4 vehicles are safer in a crash, their owners may be placing themselves and other road users at increased risk of injury."

Concern

But she said the overall levels of law-breaking were also a "major concern".

Almost one in six drivers in total did not wear seat belts, while one in 40 used mobile phones.

But the Society of Motor Manufacturers and Traders said the study was "weak".

A spokesman said: "To assume that drivers are not using seat belts and talking on mobile phones because they feel safer in 4x4s is a huge leap in logic and is not backed by any scientific evidence."

Another report in the BMJ also strengthened the link between the risk of driving while feeling sleepy.

In a survey of 13,299 middle-aged drivers, researchers found people who drove while sleepy a few times a year were 1.5 times more likely to be involved in a serious crash than those who did not drive while sleepy.

Those who said they drove while sleepy each month were nearly three times more likely to have an accident, the team from the Victor Segalen University in France said.

Roger Vincent, of the Royal Society for the Prevention of Accidents, said: "There are still too many people taking unnecessary risks on the roads. We must get better."